Field of the Invention
The present invention relates to a vibration actuator for frictionally driving a driven member by bringing the driven member into contact with a vibrating body, and an image forming apparatus mounting the vibration actuator thereon.
Description of the Related Art
Vibration actuators are put into practical use as motors for driving a driven member (hereinafter referred to as “the moving body”) in various precision machines, or the like, because of their characteristics of large torque at low speed, being small and light, having high responsiveness, etc.
A cross-sectional view schematically showing a structure of the conventional vibration actuator is shown in FIG. 7. Further, a cross-sectional view and a top view, schematically showing a structure of a vibrating body 100Z constructing the vibration actuator in FIG. 7, are shown in FIG. 8. Furthermore, a cross-sectional view, schematically showing a portion of a structure of the vibrating body 100Z and a base member 500 constructing the vibration actuator of FIG. 7, is shown in FIG. 9.
The vibration actuator in FIG. 7 includes a vibrating body 100Z, a moving body 180 which is a driven member that frictionally contacts to the vibrating body 100Z, a pressurization mechanism 300 that brings the moving body 180 into pressure contact to the vibrating body 100Z to output rotation of the moving body 180, and a shaft 400.
In FIG. 7, the vibrating body 100Z, the moving body 180, and the pressurization mechanism 300, each of which has a circular shape, are attached to a base member 500 with the shaft 400 as a central axis.
The pressurization mechanism 300 comprises a damping rubber 301, a pressurization spring retaining member 302, a pressurization spring 303, and a pressurization spring fixing member 304. The base member 500 includes a bearing 520.
As shown in FIG. 8 and FIG. 9, the vibrating body 100Z has an elastic body 110, a piezoelectric device 120 which is bonded as an electro-mechanical energy conversion device to the elastic body 110, and a flexible printed board 160 for applying an AC voltage as a driving voltage to the piezoelectric device 120.
The elastic body 110 has a structure in which an attachment portion 113, a connection portion 112, and a vibration portion 111 are integrally formed from an inner diameter side toward an outer diameter side, wherein the connection portion 112 supports the vibration portion 111 on the basis of the attachment portion 113.
The elastic body 110 is fixed to the base member 500, by a fixing screw 510, through a fixing hole 140 provided at the attachment portion 113.
The vibration portion 111 has a plurality of grooves 131 which are formed in a circumferential direction at regular intervals and extend in a radial direction, that is, a radial direction of the vibrating body 100Z, and has a plurality of protrusions 130 for expanding vibration displacement, which are formed by the plurality of grooves 131.
The attachment portion 113 is a portion for fixing the vibrating body 100Z to the base member 500.
As shown in FIG. 9, by the flexible printed board 160 bonded to a face of the piezoelectric device 120 opposite to a face to which the vibrating body 100Z is bonded, an AC voltage is applied to the piezoelectric device 120. By this means, in the vibrating body 100Z, flexural vibration with a predetermined order m (where m is a natural number) is excited as driving vibration, and the moving body 180 is frictionally driven and rotated by motion (including circular motion and elliptic motion) caused at a portion contacting to the moving body 180, of the vibrating body 100Z.
It should be noted that, as is clear from FIG. 7, a shaft 400 (not shown in FIG. 9) is located at an inner diameter side of the base member 500 in FIG. 9.
A structure has been proposed where a base material (a material) comprises ferrous metal such as stainless steel, and, as shown in FIG. 9, a nitrided layer 152 is formed on a contact face (a friction face) of the elastic body 110, contacting with the moving body 180 (see Japanese Laid-Open Patent Publication (Kokai) No. 2004-80947, Japanese Laid-Open Patent Publication (Kokai) No. 2009-55779, Japanese Patent No. 4976804, and Japanese Patent No. 5236160, for example). The nitrided layer 152 formed on the contact face improves abrasion resistance to the moving body, and contributes to improvement of durability of the vibration actuator.
However, when the surface of the elastic body 110 is subjected to nitriding treatment, a nitrided layer 152 which has unstable conductive property is also formed at a portion other than the contact face contacting with the moving body 180. In this case, a driving voltage to be applied to the piezoelectric device 120 is not stable; and hence, a problem that drive performance of the vibration actuator may become unstable occurs.